Field of the Invention
The present invention relates to an electric motor and more particularly, to a submersible electric motor of a type in which a liquid such as water is hermetically confined in the interior thereof.
There are certain submersible electric motors of known types in which a liquid is contained within the interior in a hermetically sealed manner so as to improve lubrication and cooling of bearings. Typical submersible motors are disclosed in, for example U.S. Pat. No. 3,050,645, Japanese Patent Laid-Open No. 9607/1974 and Japanese Patent Laid-Open No. 72611/1973. In these known types, a shaft projects from the top of the frame thereof and a diaphragm, acting as a pressure-control member, is disposed at the lower portion of the frame. The diaphragm includes an elastic member and, as pressure within the submersible motor increases, the diaphragm is deformed so as to increase the volume of the interior of the motor.
Generally, a variation in a diaphragm volume is determined in terms of the expansion of the liquid contained in a motor since the liquid contained in a motor expands as the temperature rises. The shaft is also rotatably supported on the frame by an upper sleeve bearing and a lower sleeve bearing, with the upper and lower sleeve bearings being lubricated and cooled with a liquid contained in the motor. Thus, it is critical that the upper sleeve bearing and the lower sleeve bearing be continuously submerged in the liquid. A common method of charging a liquid into the interior of a submersible motor is to drill a hole in the top of the frame for charging the liquid therethrough. While this type of submersible motor is operating, the upper sleeve bearing is heated, thus causing, for example, a problem of breakage. Various experiments have been conducted to determine the causes of this problem as described below.
More particularly, an interior of the submersible motor is not completely filled with the liquid since air remains in the lower portion of its rotor and the recesses of the frame. The residual air travels upwardly due to centrifugal force or vibration caused by the operation of the submersible motor. Accordingly, the air reaches the top and the upper sleeve bearing which has first to be submerged in the contained liquid is thus exposed above the liquid level, thereby making it impossible to carry out proper lubrication and cooling.
In a case where water is used as the contained liquid, the shaft and the rotor of the submersible motor come into contact with the water and are oxidized, thus generating hydrogen gas. The hydrogen gas comes to the upper portion of the submersible motor similar to the air. The quantity of hydrogen gas generated increases with the passage of time during storage or operation of the submersible motor. The hydrogen gas raises the pressure within the submersible motor and simultaneously deforms the diaphragm. When the diaphragm is deformed, the volume of the interior of the submersible motor is increased in proportion to the amount of deformation, and the liquid level in the submersible motor is lowered. Consequently, since the liquid level falls, the upper sleeve bearing is exposed above the liquid surface, thereby resulting in insufficient lubrication and cooling.